Optimizing compilers permit efficient object code to be emitted given a particular piece of source code to be compiled. Source code that includes a store operation where the variable stored has a constant value or is not later used in the source code execution path may be the subject of optimization in compilers. Such an optimization is known as a dead store elimination optimization.
Dead store elimination is a well-understood optimization. Compiler optimizations, such as constant propagation, which propagate constant values forward in compiled code, often result in opportunities for store elimination. Such compiler optimizations permit the removal of code for such store operations if, after the constant propagation, the compiler can determine that the variable subject of the store operation is not subsequently used. Removing the code for such a store operation reduces execution path length in the compiled computer code.
If the scope of the code optimization is intraprocedural only restricted to be (within the bounds of a procedure or function) then it is not possible to eliminate code for a store operation if the variable subject to the operation is an external (global) variable. In such a case the compiler cannot determine if the variable is used in the execution path that follows execution of the function and is therefore not able to carry out a dead store elimination optimization. The function is potentially called in other procedures or functions. Although a particular variable may not be modified or used in the function itself, the variable may be “live” in one of the procedures or functions that calls the function.
To carry out a dead store elimination optimization for an external variable it is therefore necessary for the optimizing compiler to carry out an interprocedural dead code elimination analysis. One prior art approach to the interprocedural dead code elimination is to carry out a data flow analysis for the entire code being compiled to determine future uses of a variable subject to a possible dead code elimination optimization. Such an approach typically involves an overhead cost which makes its use in optimizing compilers prohibitive.
It is therefore desirable to have a computer system which carries out the interprocedural dead store elimination in an optimizing compiler without the requirement of a resource intensive data flow analysis step.